Hi,
Welcome to 240 ownership and the forum, great car choice
You can lower a 240 cheaply and easily with 40mm lowering springs without running into any issues with either suspension geometry or scraping speed bumps. Depending on where you are in London a friend has almost exactly the same car as you with 40mm lowering springs fitted, which you might want to have a look at.
Engine/power wise, you’re probably best off making sure everything is working properly as a start- the cam is probably an A cam, so not much scope to improve there- a V cam has slightly more lift but not enough to make much difference. You might find a k or h cam if you’re lucky. A Weber carb kit does exist, although tuning a n/a car costs quite a lot for the power gained. Obviously I’m going to say turbo swap it…
Throw out any questions you have, there’s a lot of knowledge on this forum and someone will know the answer to anything you ask….
Cheers